This invention relates to a private branch exchange system in which a terminal in the process of communicating can be moved between the same or different communication lines, wherein the communication is temporarily put on hold during this movement and then communication with the communicating party is resumed by the terminal after it has been moved.
In an ISDN standardized by the CCITT (International Telegraph and Telephone Consultative Committee), an information channel (B channel) and a signal channel (D channel) generally are completely isolated and therefore it is possible to exchange a wide variety of control signals. Accordingly, a diversity of services can be developed by the various types of control signals.
With an ISDN basic interface stipulated by the I-Series recommendations of the CCITT, a bus wiring system in which a maximum of eight terminals are capable of being connected to the same line of a four-wire system is employed in the S-point interface and T-point interface, which are the stipulated points of a user network interface. Accordingly, when a connector such as a modular jack or the like is used at the connection to the terminal, various terminals such as telephones and facsimile machines can be attached and detached at a single touch. Further, Recommendation Q.931 of the CCITT stipulates a service for moving devices during communication, which service uses an interruption message and a resumption message.
FIG. 12 illustrates a common network using such as ISDN. A private branch exchange (PBX) 801 is connected to a terminal 810 via an S-point interface and a digital communication line 821, and to an ISDN 830 via a T-point interface and a digital communication line 822. The S-point interface is of the point-multipoint type. The block 811 shown by the dashed line indicates that there is connector in the bus wiring but that no terminal has been connected to it.
FIG. 13 is a flowchart for describing an interruption/resumption sequence of the terminal 810 in FIG. 12, and FIG. 14 is a flowchart for describing an interruption/resumption sequence of the PBX 801 in FIG. 12. The service for moving a device in the process of communicating is started when the terminal 810 transmits an interruption message to the PBX 801.
In FIG. 13, the terminal 810 is in communication with the ISDN 830 via the PBX 801 at step 900. When, say, an interrupt key (not shown) is pressed at step 901 under these conditions, the terminal 810 transmits an interruption message through the PBX 801 at step 902 and assumes an interruption-request state at step 903. In this case, the terminal 810 transmits the interruption message, which contains a call identifying information element as an option. The call identifying information element is of the kind shown in FIG. 11.
Meanwhile, in FIG. 14, the PBX 801 is in a communicating state at step 1000. When the interruption message from the terminal 810 is received at step 1001, the PBX 801 attains an interruption-request state at step 1002. In a case where the call identifying information element of the kind shown in FIG. 11 is verified at step 1006, an interruption verification message is transmitted to the terminal 810 at step 1007. Then, at step 1008, the interrupt call and the call identifying information element are stored in memory and the call on the side communicating party is put on hold on the side of the ISDN 830 at step 1008. The ISDN 830 is notified of fact that the terminal 810 issued the interrupt, and the terminal 810 becomes idle at step 1009.
When the terminal 810 receives the interruption verification message from the PBX 801 at step 904 in FIG. 13, the terminal 810 becomes idle at step 907. Accordingly, the terminal 810 is capable of being moved, while the holding state is in effect, to the position of the block 811 indicated by the dashed line in FIG. 12.
In a case where the call identifying information element is insufficient, i.e., when the call identifying information is inadequate and does not possess sufficient information relating to resetting (step 1003), the PBX 801 transmits an interruption refusal message to the terminal 810 at step 1004 and a transition is made from the interruption-request state to a communication-in-progress state at step 1005, as shown in FIG. 14. For example, there are cases where the call identifying information duplicates a number utilized by another interrupt call. Upon receiving the interruption refusal message at step 905 in FIG. 13, the terminal 810 makes a transition from the interruption-request state to the communication-in-progress state at step 906.
When a resume key (not shown) on the terminal 810 that has been moved to the block 811 is pressed at step 908, the terminal 810 sends the PBX 801 (step 909) a resumption message containing a call identifying information element the same as that which prevailed when the interruption message was sent, and the terminal 810 makes a transition to a resumption-request state at step 910.
Upon receiving the resumption message at step 1010 in FIG. 14, the PBX 801 makes a transition to the resumption-request state at step 1011. The PBX then compares the call identifying information elements of the interruption message and resumption message and, if they coincide, releases the stored call identifying information elements at step 1015 and sends a resumption verification message to the terminal 810 at step 1016. At the communicating party is released from the holding state and the PBX makes a transition to the communicating state at step 1017. The resumption verification message contains a channel identifier which displays a message reading "Desired channel exists. Change to other channel not possible." in order to designate the S-point information channel at the moment of hold.
Upon receiving the resumption verification message at step 911 in FIG. 13, the terminal 810 undergoes a transition to the communicating mode at step 914. Accordingly, the communicating party is released from the holding state and communication again becomes possible between the ISDN 830 and the terminal 810.
When the call identifying information elements of the interruption message and resumption message do not coincide at step 1012 in FIG. 14, the PBX 801 sends a resumption refusal message to the terminal 810 at step 1013, the stored call identifying information elements are released, an interruption message is transmitted to the ISDN 830, and a transition is made to the idle state at step 1014. Upon receiving the resumption refusal message at step 912 in FIG. 13, the terminal 810 similarly makes a transition to the idle state at step 913.
In the conventional private branch exchange described above, movement of the device during communication can be carried out only in the same S-point interface. Consequently, in a case where the PBX 801 has a plurality of S-point interfaces, a problem 0 which arises is that communication will be lost if the terminal 810 is mistakenly connected to another digital communication line.
In addition, in the conventional private branch exchange described above, selection of the call identifying information is left to the individual moving the communication device. As a consequence, (1) the individual moving the communication device must not duplicate the call identifying information of another individual moving a communication device, and (2) if there is no call identifying information, the number of interruption calls is limited to one. These limitations can be an annoyance to the individual moving the communication device.